In solid-state lasers (SSL), optical pumping generates a large amount of heat within a laser medium and increases its temperature. Continuous operation of the laser, therefore, requires removal of the waste heat by cooling selected surfaces of the laser medium. Because SSL media typically have a low thermal conductivity, a significant thermal gradient is created between the hot interior and the cooled outer surfaces. This causes a gradient in the index of refraction, mechanical stresses, depolarization, detuning, and other effects, with likely consequences of degraded beam quality, reduced laser power, and possibly a fracture of the SSL medium. Such effects present a major challenge to scaling of SSLs to high-average power (HAP). Pumping by semiconductor laser diodes, which was introduced in the 1990's, greatly reduces the amount of waste heat and paves the way for development of a high-average power (HAP) SSL with good beam quality (BQ). Such lasers are expected to make practical new industrial processes such as precision laser machining with applications ranging from deep penetration welding to processing of aerospace materials.
It has been long recognized that optical distortions caused by transverse temperature gradients (i.e., perpendicular to laser beam axis) degrade beam quality. A class of SSL known as “disk lasers” avoids transverse temperature gradients because waste heat is extracted from the disk in the direction parallel to the laser beam axis. Because of this one-dimensional heat flow, solid-state disk lasers (SSDL) enjoy inherently low susceptibility to thermal lensing and stress birefringence. In addition, their large, round aperture reduces diffraction and beam clipping losses experienced by other SSL configurations. SSDL may use “transmissive” disks having antireflective coatings on each face or “reflective” disks having an antireflective coating on one face and highly reflective coating on the other. In a transmissive disk, waste heat is removed by a coolant (usually gas) flowing through the laser beam path. In a reflective disk, also known as an “active mirror amplifier” (AMA), the back surface of the disk is available for liquid cooling, which can be applied more uniformly, and can easily handle heat fluxes. Both disk laser types (transmissive and reflective) have been in development since the late 1960s, especially as amplifiers for giant pulse lasers for inertial confinement fusion.
The AMA was originally disclosed by Almasi et al. in U.S. Pat. No. 3,631,362 (1971) and has shown effective reduction of transverse temperature gradients and demonstrated the generation of a laser output with very good beam quality. See, for example, J. Abate et al., “Active Mirror: A large-aperture Medium Repetition Rate Nd: Glass Amplifier,” Appl. Opt. Vol. 20, no. 2, 351–361 (1981) and D. C. Brown et al., “Active-mirror Amplifier: Progress and Prospects,” IEEE J. of Quant. Electr., vol. 17, no. 9, 1755–1765 (1981).
In the classical AMA concept, a large aspect ratio, edge-suspended, Nd-Glass disk (or slab) several centimeters thick is pumped by flashlamps and liquid-cooled on the back face. However, this device is not suitable for operation at high-average power because of poor heat removal and resulting thermo-mechanical distortion of the edge-suspended disk. Previous attempts to mitigate these problems and increase the average power output of an SSDL were met with encouraging but limited results. In recent years, the SSDL concept has been revived in the form of a “thin disk laser” AMA introduced by Brauch et al. in U.S. Pat. No. 5,553,088. The thin disk laser uses a gain medium disk which is several millimeters in diameter and 200–400 micrometers in thickness soldered to a heat sink. See, for example, A. Giesen et al., “Scalable concept for diode-pumped high-power lasers,” Appl. Phys. B vol. 58, 365–372 (1994). The diode-pumped Yb:YAG thin disk laser has demonstrated laser outputs approaching 1 kW average power and with beam quality around 12 times the diffraction limit. See, for example, C. Stewen et al., “1-kW CW Thin Disk Laser,” IEEE J. of Selected Topics in Quant. Electr., vol. 6, no. 4, 650–657 (July/August 2000). Another variant of the thin disk laser can be found in L. Zapata et al., “Composite Thin-Disk Laser Scalable To 100 kW Average Power Output and Beyond,” in Technical Digest from the Solid-State and Diode Laser Technology Review held in Albuquerque, N. Mex., Jun. 5–8, 2000.
The applicant's U.S. Pat. No. 6,339,605 titled “Active Mirror Amplifier System and Method for a High-Average Power Laser System”, hereby incorporated by reference, discloses a new SSDL concept, which is suitable for operation at high-average power. The invention uses a large aperture laser gain medium disk about 2.5 mm in thickness and with a diameter typically between 5 cm and 15 cm mounted on a rigid, cooled substrate. Note that the disk thickness in this SSDL concept is about 10 times less than in the classical SSDL and about 10 times more than in the thin disk laser. The substrate contains a heat exchanger and microchannels on the surface facing the laser medium disk. The disk is attached to the substrate by a hydrostatic pressure differential between the surrounding atmosphere and the gas or liquid medium in the microchannels. This novel approach permits thermal expansion of the laser medium disk in the transverse direction while maintaining a thermally loaded disk in a flat condition. The teachings of this patent provide numerous advantages over prior art SSL devices and allow generation of near diffraction limited laser output at very high-average power from a relatively compact device.
The above-mentioned U.S. Pat. No. 6,339,605 also teaches two principal methods for providing pump radiation into the SSDL disk, namely 1) through the large(front or back) face of the disk, or 2) through the sides (edges) of the disk. Side-pumping takes advantage of the long absorption path (approximately same dimension as the diameter of the gain medium disk), which permits doping the disk with a reduced concentration of lasant ions. When quasi-3 level lasing ions are used, reduced doping associated with side-pumped SSDL conveniently reduces pump radiation intensity required to induce laser medium transparency at laser wavelength. In addition, long absorption in a side-pumped SSDL permits the use of laser ions with small pump cross-sections that may be otherwise impractical to use for face-pumping.
A side-pumped SSDL is disclosed in U.S. Pat. No. 6,625,193 granted to the Applicant and in Applicant's co-pending U.S. Pat. application Ser. No. 10/662,063 filed on Sep. 12, 2003.
While side-pumping is a suitable method for delivering pump radiation, several associated technical challenges still need to be overcome, such as: 1) delivering and concentrating pump radiation into the relatively small area around the disk perimeter; 2) preventing overheating of the disk in the areas where the pump radiation is injected; 3) generating uniform laser gain over the SSDL aperture; and 4) avoiding laser gain depletion by amplified spontaneous emission (ASE) and parasitic oscillations. The significance of these challenges and related solutions disclosed in the prior art are discussed below.
Concentration of Pump Radiation
Modern SSL are optically pumped by semiconductor lasers commonly known as laser diodes. Because each laser diode produces a relatively small optical output (up to a few watts), pumping of SSL for HAP requires the combined output of a great many laser diodes (typically in quantities ranging from hundreds to hundreds of thousands). For this purpose the diodes are arranged in one-dimensional arrays often called “bars” containing about 10 to 100 diodes and two-dimensional arrays often called “stacks” containing several hundred to several thousand diodes. Bars are frequently mounted on water-cooled heat exchangers. Stacks are typically produced by stacking up to about 100 bars and mounting them onto a heat exchanger or by stacking up to about 20 bars already mounted on their individual heat exchangers. A good example of commercially available stacks is the Model SDL-3233-MD available from SDL, Inc., of San Jose, Calif., which can produce 200 microsecond-long optical pulses with a total output of 960 watts at a maximum 20% duty factor. SSL for HAP may require a combined power of multiple diode bars to produce desired pumping effect in the laser gain medium. Regardless of the grouping configuration, individual laser diodes emit optical radiation from a surface, which is several micrometers high and on the order of 100 micrometers wide. As a result, the beamlet of radiation emitted from this surface is highly asymmetric: highly divergent in a direction of the 1 .mu.m dimension (so called “fast axis”) and moderately divergent in the transverse dimension (so-called “slow axis”). This situation is illustrated in FIG. 2. Typical fast axis divergence angles (full-width at half-maximum intensity) range from 30 to 60 degrees, while slow axis divergence angles typically range from 8 to 12 degrees. Optical radiation from an array of diodes has similar properties. High divergence in the fast axis makes it more challenging to harness the emitted power of diode arrays for use in many applications of practical interest. Some manufacturers incorporate microlenses in their laser diode arrays to reduce fast axis divergence to as little as a few degrees. An example of such a product is the lensed diode array Model LAR23P500 available from Industrial Microphotonics Company in St. Charles, Mo., which includes microlenses which reduce fast axis divergence to less than three degrees.
The intensity of the optical output of diode arrays (lensed or unlensed) is sometimes insufficient to pump a SSL gain medium to inversion, and the radiation must therefore be further concentrated. In previously developed systems, optical trains with multiple reflecting and/or refracting elements have been used. See, for example, F. Daiminger et al., “High-power Laser Diodes, Laser Diode Modules And Their Applications,” SPIE volume 3682, pages 13–23, 1998. Another approach disclosed by Beach et al., in U.S. Pat. No. 5,307,430 uses a lensing duct generally configured as a tapered rod of rectangular cross-section made of a material optically transparent at laser pump wavelength. Operation of this device relies on the combined effect of lensing at the curved input surface and channeling by total internal reflection. Light is concentrated as it travels from the larger area input end of the duct to the smaller area exit end. Yet another approach for concentrating pump radiation disclosed by Beach et al. in U.S. Pat. No. 6,160,939 uses a combination of a lens and a hollow tapered duct with highly reflective internal surfaces.
Thermal Control of Disk Perimeter
The surfaces of the laser gain medium that receive pump radiation are susceptible to overheating and, as a result, to excessive thermal stresses. Experience with end-pumped rod lasers shows that a composite rod having a section of doped and undoped laser material provides improved thermal control and concomitant reduction in thermal stresses. See, for example, R. J. Beach et al., “High-Average Power Diode-pumped Yb:YAG Lasers,” UCRL-JC-133848 available from the Technical Information Department of the Lawrence Livermore National Laboratory, U.S. Department of Energy. A suitable method for constructing composite optical materials of many different configurations is disclosed by Meissner in U.S. Pat. No. 5,846,634. Another method suitable for forming glass optics involves casting and fusion bonding doped, undoped, or differently doped sections. Such a method is available from Kigre Inc., Hilton Island, S.C. Yet another method suitable for construction of composite polycrystalline materials is available from Baikowski International, Corp. of Charlotte, N.C.
Uniform Laser Gain Across the Aperture
Due to the exponential absorption of pump radiation, portions of the laser gain medium that are closer to the pump source are susceptible to being pumped more intensely than portions that are further away. Non-uniform deposition of pump energy can result in non-uniform gain. Gain non-uniformities across the laser beam aperture (normal to the laser beam axis) are highly undesirable as they lead to degradation of beam quality. In prior art devices, non-uniform pump absorption has been compensated for in a side-pumped rod laser by the gain medium being fabricated with a radially varying level of doping. Gain uniformity in a side-pumped SSDL can be also improved by appropriately arranging pump diodes around the perimeter of the laser disk. Suitable techniques for this purpose are disclosed by the Applicant in a co-pending U.S. patent application Ser. No. 10/441,373 filed on May 19, 2003. An alternate approach to achieving uniform gain is known as “bleach-wave pumping” which has been proposed by W. Krupke in “Ground-state Depleted Solid-state Lasers: Principles, Characteristics and Scaling,” Opt. and Quant. Electronics, vol. 22, S1–S22 (1990). Bleach wave pumping largely depletes the atoms in the ground energy state and pumps them into higher energy states. Achieving high uniformity of gain becomes even more challenging as the incident laser beam causes saturation-induced change in the spatial distribution of gain. Thus, the weaker portions of the signal are amplified relatively more than the stronger portions because they saturate the medium to a lesser degree.
Amplified Spontaneous Emission (ASE)
Amplified Spontaneous Emission (ASE) is a phenomenon wherein spontaneously emitted photons traverse the laser gain medium and are amplified before they exit the gain medium. The favorable condition for ASE is a combination of high gain and a long path for the spontaneously emitted photons. ASE depopulates the upper energy level in an excited laser gain medium and robs the laser of its power. Furthermore, reflection of ASE photons at gain medium boundaries may provide feedback for parasitic oscillations that aggravate the loss of laser power. If unchecked, ASE may become large enough to deplete the upper level inversion in high-gain laser amplifiers. Experimental data suggests that in q-switched rod amplifiers ASE loss becomes significant when the product of gain and length becomes larger than 2.25, and parasitic oscillation loss becomes significant when the product is larger than 3.69. See, for example, N. P. Barnes et al., “Amplified Spontaneous Emission—Application to Nd:YAG Lasers,” IEEE J. of Quant. Electr., vol. 35, no. 1 (January 2000). Continuous wave (CW) or quasi-CW lasers are less susceptible to ASE losses because their upper level population (and hence their gain) is clamped.
A traditional method for controlling ASE losses to an acceptable level is disclosed, for example, by Powell et al. in U.S. Pat. No. 4,849,036. This method involves cladding selected surfaces of the laser gain medium with a material that can efficiently absorb ASE radiation. In particular, it is well known in the art that each of divalent cobalt and divalent samarium ions can absorb ASE in Nd laser operating around 1.06 micrometer wavelength. In addition, divalent cobalt ions can absorb ASE radiation in an Er laser operating at a 1.54 micrometer wavelength. To reduce the reflection of ASE rays at the cladding junction, the cladding material must have an index of refraction at the laser wavelength that is closely matched to that of the laser gain medium. Recently, another method for ASE loss control was introduced. In this method, ASE rays are channeled out of selected laser gain medium surfaces into a trap from which they are prevented from returning. See, for example, R. J. Beach et al., “High-average Power Diode-pumped Yb:YAG Lasers,” supra.
Materials and Methods for Low Waste Heat
To operate a SSL at HAP, it is critical to reduce as much as possible the Stokes shift (difference between the lasing wavelength and the pump wavelength), which is the leading energy loss mechanism contributing to production of waste heat. Waste heat is deposited into the gain medium where it is responsible for thermal lensing, mechanical stresses, depolarization, degradation of beam quality (BQ), loss of laser power, and (in extreme cases) thermal fracture. Consequently, when pumping a HAP SSL, it is highly desirable to use pump absorption features in proximity to the laser emission line.
The most important lasant ions for a HAP SSL operating near 1-micrometer wavelength are trivalent neodymium (Nd3+) and trivalent ytterbium (Yb3+). In addition, trivalent erbium (Er3+), lasing at a 1.54 micrometer wavelength, is important for applications requiring increased eye damage threshold (also known as “eye-safe”). Each Nd, Yb, and Er can be doped into a variety of crystalline, polycrystalline and amorphous host materials.
A side-pumped SSDL disclosed herein makes it possible to reduce the Stokes shift in many important materials and allow Nd and Yb lasers to operate more efficiently. In particular, neodymium ion Nd3+ is traditionally pumped by diodes on the 808-nm absorption line that has a large cross-section. In contrast, pumping Nd on a weaker absorption feature around 885 nm deposits energy directly into the upper lasing level. Direct pumping improves Stokes efficiency by nearly 10% and entirely avoids the quantum efficiency loss (estimated at 5% in Nd) associated with energy transfer from the pump band to the upper lasing level. A side-pumped disk is amenable to direct pumping of Nd despite having a low absorption cross-section and narrow width of absorption feature at a wavelength of around 885 nm.
Ytterbium is characterized by a Stokes shift several times smaller than for Neodymium. Yb:YAG and Yb:GGG are traditionally pumped at the broad absorption feature around 941 nm. A more efficient approach is to pump Yb at the zero-phonon line around 970 nm, which offers a smaller Stokes shift and deposits energy directly into the upper laser level. A side-pumped disk laser is amenable to pumping ytterbium despite its rather low absorption cross-sections in many host materials of practical interest, namely YAG, GGG, and glass. Low absorption cross-section makes it more problematic to absorb pump energy in a short distance, as may be desirable for face-pumping of disk and slab lasers or side-pumping rod lasers. A short absorption path in combination with small absorption cross-section necessitates high doping which, in turn, requires very high pump intensities to overcome re-absorption of laser radiation by the ground energy state. This problem is resolved with the side-pumped disk of subject invention, which offers a long absorption path.
Polycrystalline Host Materials: Host materials such as YAG and GGG for use in SSL have been traditionally produced in a single-crystal form typically using the Czochraski method. However, production of large crystals required for HAP SSL is a slow and expensive process. Furthermore, such crystals are very limited in size, contain undesired inclusions, frequently can be produced with uniform doping, and are polluted by trace elements from the melt container. Recently, polycrystalline YAG and other polycrystalline garnets have emerged as viable replacements for single-crystal materials, offering large size products at reduced cost, improved quality, and improved fracture resistance. Such materials have been developed in Japan by Konoshima Chemical Company and are marketed in the US by Baikowski International Corp. of Charlotte, N.C.
Athermal Glass
Waste heat deposited into a SSL gain medium causes temperature changes which result in thermo-optic distortions that may affect the optical phase-front of the amplified laser beam and degrade its beam quality. Such distortions include thermal expansion, change to the index of refraction (n), and thermal stress-induced birefringence. Materials have been developed that reduce some of these effects. In particular, a glass composition known as athermal glass compensates for the positive coefficient of thermal expansion by a negative coefficient of change to the refractive index (dn/dt) to produce a very low thermal coefficient of optical path. Glass with athermal properties is sold by Kigre Inc. of Hilton Head Island, S.C. under designations Q-98 and Q-100; and by Schott Glass Technologies, Inc., in Duryea, Pa. under designation LG-760.